Simultaneous air–water backwashing with millibubbles under low airflow conditions: enhancing deposit removal and mitigating mudball precursor formation in rapid sand filters

Graphical abstract showing simultaneous millibubble air-water backwashing features. A chart compares residual deposits among three methods, while a schematic illustrates rising bubbles growing through the filter bed, creating wake turbulence and surface jet flow. Rapid sand filters require effective backwashing to maintain the cleanliness of filter media. This study investigated simultaneous air–water backwashing with millibubbles generated at a significantly reduced airflow rate (0.03 m3/(m2·min), approximately one-tenth the conventional rate). The performance of the proposed approach was compared with those of standard water-only and sequential air–water backwashing over 16 filtration–backwashing cycles. The millibubble-assisted air–water backwashing achieved higher suspended solid removal efficiency and lower cumulative residual deposits than the water-only backwashing, while demonstrating cleaning performance comparable to, or slightly higher than, that of conventional air–water backwashing. Bubble-induced hydrodynamic interactions enhance particle detachment, thereby mitigating mudball precursor formation. The low-airflow condition is expected to reduce the risk of media loss and bed disturbance, which is particularly desired in pressure filters where direct observation is restricted. These findings demonstrate a stable cleaning strategy for sustainable water treatment.